Performance augmentation of natural draft cooling towers

ABSTRACT

A method and apparatus are provided for enhancing the performance capability of an existing natural draft cooling tower( 13 ). The cooling tower includes a structure ( 14 ) defining an open-topped internal passage ( 15 ) ofcircular cross section for the upward flow of air from one or more inlets ( 16 ) at the base of the structure. The method includes steps of providing within the passage ( 15 ), an impeller ( 27 ) adapted when mtated at a specified speed about an upright axis of rotation ( 28 ) centrally located in the passage to increase the overall flow rate of air in the passage beyond the overall flow rate obtainable in the same operatiny conditions by natural draft alone and provides support means for said impeller and drive means capable of rotating the impeller at the specified speed. The impeller ( 27 ) is supported by the supporn means above heat transfer means ( 19 ) (which may be a wetted packiny structure) of the cooling tower ( 13 ).

FIELD OF THE INVENTION

This invention relates to a method and apparatus for augmenting theperformance of natural draft cooling towers, a cooling tower withvariable cooling capacity and a method of enhancing the performance of apower generating system fitted with a natural draft cooling tower.

BACKGROUND

Cooling towers are heat exchangers of a type widely used for rejectionof low grade heat to atmosphere, in electricity generation, airconditioning installations and the like. This invention is directed tolarge capacity natural draft cooling towers such as those used inelectric power generation. These are built in very large sizes, forlowering the temperature of condenser cooling water. Such cooling towershave been built with tower heights over 100 m and base diameters over 90m. Such cooling towers may have design cooling water flow rates in theorder 5 to 50 m³/s or more.

In a natural draft cooling tower for this application, air flow isinduced in a hollow chimney-like tower by the density difference betweencool air entering the bottom of the tower and warm air leaving the top,due to heat transfer from the water being cooled, which is passedthrough the interior of the tower. Most such cooling towers are “wet”types, in which there is direct contact between water being cooled andthe flowing air, so that a proportion of the water evaporates. The toweritself is typically a hollow, open-topped shell of reinforced concretewith an upright axis of symmetry and circular cross-section, the shellwall having a necked, hyperbolic shape when seen in meridiancross-section. Openings at the base of the tower structure enableingress of air. The convergent-divergent shape aids in inducing thenatural draft.

“Dry” type natural draft cooling towers are also known, in which thefluid to be cooled remains isolated from the cooling air, together withhybrid wet/dry types.

Forced draft cooling towers are also known, in which the air flow isproduced by fans. In these devices, there is generally no true towerstructure similar to the open-topped shell of a large natural draftcooling tower, because the fans replace the chimney effect of thenatural draft cooling towers.

It is also known to provide large cooling towers, with hyperbolic shellsas in the large natural draft cooling towers, but with forced draft orfan augmentation of natural draft. These use multiple, high-speed fans,and are comparatively complex and expensive by comparison with simplenatural draft cooling towers. See for example U.S. Pat. 3,903,212 andBritish Patent 1455544. Further, such arrangements are generally notsuitable for modification of existing installations.

Once a natural draft cooling tower has been built, its performance for agiven set of atmospheric conditions (temperature, humidity and wind),water flow rate and temperature is essentially fixed. Cooling towers aredesigned to achieve a specified performance at specified “design-point”conditions. Only physical changes to the cooling tower arrangement wouldchange its performance characteristics. For example, changes to the”packing” of a wet-type natural draft cooling tower (i.e. thelarge-surface-area structure at its base which is wetted by the fluid tobe cooled and which extends the residence time of that water in thecooling tower, can affect cooling capacity to a significant degree.However, this is not done as a routine matter: if an improved packingdesign is developed, it may be implemented as a “one off” improvement ifcost-effective. Generally, the options for economically improving orvarying existing cooling tower performance are very limited.

It is known that improving cooling tower performance (i.e. the abilityto extract an increased quantity of waste heat in a given time) can leadto improved overall efficiency of a steam plant's conversion of heat toelectric power and/or to increases in power output in particularconditions. Cost-effective methods of improvement are desired,particularly in already-existing plant. The present invention addressesthis desire. Equivalent considerations can apply in other industrieswhere large natural draft cooling towers are used.

Large natural draft cooling towers are high-capital-cost, long-lifefixed constructions, and it is desirable that improvements be obtainablewithout major modifications, particularly to the main tower structure.The method and apparatus of the present invention are applicable to theimprovement of existing natural draft cooling towers, as well as to newcooling towers. The modification of existing towers may in fact be themain application area of the invention.

The present invention also provides for a degree of readily controllablevariation of cooling tower performance to be obtained.

SUMMARY OF THE INVENTION

Surprisingly, it has been found in suitable cases to be feasible andworthwhile to economically enhance the performance of large naturaldraft cooling towers having a circular cross-section by providing foraugmentation of the natural draft by a rotating impeller (fan) withinthe cooling tower structure which in use spans substantially the wholeinternal diameter of the cooling tower, or a large proportion thereof.Cooling towers to which the invention is applicable include, inparticular, those of such large size that the use of fans to provide oraugment the draft of air has been considered impractical or tooexpensive to be cost-effective, particularly in retrofit applications.The invention is applicable to both new and existing cooling towers, butoffers in particular the possibility of upgrading an existing coolingtower without greatly affecting the packing, and requiring comparativelylimited space for new equipment inside and outside the existingstructure.

According to the invention, there is provided, in a first aspect, amethod for enhancing the performance capability of an existing naturaldraft cooling tower, wherein the cooling tower:

(a) is adapted including by size and cooling capacity in natural draftoperation for use as a natural draft cooling tower in electric powergenerating station application,

(b) includes a structure defining an internal passage of circularcross-section for the upward convectional flow of an air steam thereinfrom air inlet openings at or near a lower part of the structure to anoutlet opening at the top of the structure, and

(c) contains heat transfer means in a lower part of said passage fortransferring heat from water supplied to said cooling tower to said air,

and wherein said method includes the steps of:

providing within said passage an impeller adapted when rotated at aspecified speed about an upright axis of rotation centrally located insaid passage in a specified operating condition of said tower toincrease the flow rate of air in the passage beyond an overall flow rateobtainable in identical operating conditions by natural draft alone;

providing support means adapted for supporting said impeller within saidpassage above said heat transfer means; and

providing drive means capable of rotating said impeller at saidspecified speed.

In a second aspect of the invention, there is provided apparatus forenhancing the performance of a natural draft cooling tower, saidapparatus being adapted to use in a cooling tower that:

(a) is adapted including by size and cooling capacity in natural draftoperation for use as a natural draft cooling tower in electric powergenerating station application,

(b) includes a structure defining an internal passage of circularcross-section for the upward convectional flow of an air steam thereinfrom air inlet openings at or near a lower part of the structure to anoutlet opening at the top of the structure, and

(c) contains heat transfer means in a lower part of said passage fortransferring heat from water supplied to said cooling tower to said air,

and said apparatus including:

an impeller adapted when rotated at a specified speed about an uprightaxis of rotation centrally located in said passage in a specifiedoperating condition of said tower to increase the flow rate of air inthe passage beyond an overall flow rate obtainable in identicaloperating conditions by natural draft alone;

support means adapted for supporting said impeller within said passageabove said heat transfer means, and

drive means capable of rotating said impeller at said specified speed.

In a third aspect of the invention, there is provided a cooling towerfor cooling a liquid and having a cooling capacity variable by a user,said cooling tower:

(a) being adapted including by size and cooling capacity in naturaldraft operation for use as a natural draft cooling tower in electricpower generating station application,

(b) including a structure defining an internal passage of circularcross-section for the upward convectional flow of an air steam thereinfrom air inlet openings at or near a lower part of the structure to anoutlet opening at the top of the structure,

(c) containing heat transfer means in a lower part of said passage fortransferring heat from water supplied to said cooling tower to said air,and

(d) including apparatus as disclosed above and herein for increasing thecooling capacity of said cooling tower when in operation.

In a fourth aspect of the invention, there is provided a method forenhancing the performance of a power generation plant in which:

steam is passed through a turbine which drives an electric powergenerator and said steam is condensed in a condenser;

cooling water for said condenser is circulated through said condenserand a natural draft cooling tower;

said method including the steps of:

adding to said cooling tower apparatus for enhancing the performance ofsaid cooling tower, said apparatus being apparatus as disclosed above orherein; and

operating said apparatus.

Further preferred further features of the invention are set out in boththe appended claims and the detailed description below.

The invention will now be described in more detail, although without anyintention to limit the scope of the invention, by reference to theattached Figures, of which:

FIG. 1 is a schematic steam/water circuit diagram of a simplifiedelectric power generating installation;

FIG. 2 is a side view of a counterflow-type, “hyperbolic” natural draftcooling tower, seen in vertical cross section, the section being takenat the symmetry axis of the tower structure;

FIG. 3 is a side view of a crossflow-type, “hyperbolic” natural draftcooling tower, seen in vertical cross section, the section being takenat the symmetry axis of the tower structure;

FIG. 4 is a side view of a counterflow-type, “hyperbolic” natural draftcooling tower, embodying the invention, the cooling tower being seen invertical cross section, taken at the symmetry axis of the towerstructure;

FIG. 5 is a cooling tower embodying the invention in a form that is analternative to the form shown in FIG. 4, seen in a view that is of thesame type as FIG. 4;

FIG. 6 is a cooling tower embodying the invention in a form that is afurther alternative to the form shown in FIG. 4, seen in a view that isof the same type as FIG. 4.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a schematic diagram of the steam/water circuit 1 of agreatly-simplified electric power generating installation. A boiler 2produces steam which is led by a duct 3 to a steam turbine 4 whichdrives a generator 5. The boiler 2 may burn fossil fuel (eg coal ornatural gas) to provide heat or the heat source may be a nuclear reactor(not shown). Wet steam exiting the turbine 4 is condensed in a condenser6 and exits condenser 6 as water, which is recirculated as feedwater toboiler 2 via a feedwater pump 7.

A separate cooling water supply is provided to condenser 6 via a duct 8and exits in a hotter state via a duct 9, being pumped by cooling waterpumps 10. In some installations, a large supply of water is availablefrom a lake, river or artificial “cooling pond” for use as coolingwater. However, where this is not the case, cooling water may bedirectly recirculated as shown in FIG. 1, passing through a coolingtower 11 to lower its temperature before returning to the condenser 6via duct 8. This arrangement avoids the need for a large natural supplyof cooling water, requiring only a fixed quantity of cooling water incirculation and comparatively small amounts (typically 1% to 2% of thecooling water flow) of makeup water to compensate for evaporation lossesin the cooling tower 11.

It is to be understood that the circuit 1 is for illustrative purposesonly. In a practical power generating installation, (not shown) therewould be additional components such as economisers, superheaters, and(usually) multiple boilers and turbines and ducting to accommodate them.

The overall efficiency of circuit 1 and/or the external work done byturbine can be increased by lowering the temperature of cooling waterentering the condenser 6. The invention is directed to a method andapparatus for doing this in an existing circuit 1, by modification ofcooling tower 11. The invention can also be applied to new coolingtowers.

Cooling towers are heat exchangers of the type in which a liquid (incircuit 1, the cooling water) is passed into a space through which a gas(in circuit 1, atmospheric air) is flowing and in that space is cooledby direct contact with the cooler air and by partial evaporation. Togive sufficiently long liquid residence times and gas/liquid interfaceareas, the liquid is often sprayed into the space, falling downward orbeing splashed onto a large-surface-area fixed structure (known forexample as “packing”) at the base of the tower, finally collecting in abasin below the packing and from there leaving the cooling tower. Insmall cooling towers of the sizes used in air conditioning and similarapplications, the flow of gas is normally produced by fans, typicallyintegral with the cooling tower itself. However, in the largest coolingtowers, typical of electric power generation applications, natural draftis relied on to provide the airflow.

The following discussion is restricted to “wet-type” natural-draftcooling towers, in which the liquid to be cooled is directly exposed tothe air flowing in the tower, this being the most common type found inlarge sizes such as those typical of electric power generating stations.It is to be understood however that there is no technical obstacle toapplication of the invention in “dry-type” or hybrid wet/dry naturaldraft cooling towers.

FIG. 2 shows a counterflow-type, “hyperbolic” natural draft coolingtower 13. A reinforced-concrete wall 14, of hyperbolic shape when seenin meridian section (as in FIG. 2), defines an open-topped passage 15 ofcircular cross-section through which air flows upwardly from openings 16spaced peripherally around the base of wall 14. Because the air leavingthe top of passage 15 is hotter and less dense than the air entering atthe bottom due to heat transfer from the water in the tower 13, anatural draft is induced, as shown by arrows 100, and the hyperbolicshape of the wall 14 enhances this effect. The wall 14 shape givespassage 15 a neck 70, where the internal diameter of passage 15 is at aminimum. Hot inlet water is introduced through pipe system 17. Below adrift eliminator 18, the water is sprayed or splashed downwardly ontoand through packing 19 (for which no internal detail is shown) andcollects in a depression 20 in the base of the tower 13, finallyentering outlet pipes 21. While in the passage 15, the downwardly-movingwater is cooled by direct contact with upwardly moving (i.e.counterflowing) air and by partial evaporation.

FIG. 3 shows a crossflow-type natural draft cooling tower 22. This isgenerally similar to the counterflow-type cooling tower 13, except thatthe packing 23 (for which no internal detail is shown) is located in anannular ring formation external to, and around the base of, hyperbolicwall 102. The water to be cooled passes downward through the packing 23from pipe system 24, but the draft of air flowing into the tower 22moves generally horizontally through the packing 23, as shown by arrows101, so that there is a crossflow-type interaction between air andwater.

The present invention is applicable to both cross-flow and counter-flownatural draft cooling towers as shown (13, 22) in FIGS. 2 and 3.Typically the mean air velocity above the packing in large natural draftcooling towers is in the range 1.2 to 1.8 mls. (See Perry's ChemicalEngineers Handbook, 7th Edition, 1997, p12.21.)

FIG. 4 shows a counterflow-type cooling tower 25 similar to coolingtower 13, but to which the present invention has been applied. Withinpassage 26 defined by the tower structure 125 there is mounted animpeller 27 which can rotate about a vertical axis 28 coaxial with thepassage 26. The impeller 27 is secured to a shaft 29 (not shown) coaxialwith, and extending upwardly, through a tube 33 in a slender supporttower 30 which is mounted on foundation 31 in the water collection pond20 a of the tower 25. This requires modification of only a smallproportion of the packing 19 a. Impeller 27 has a number of blades 41extending radially outward from a hub 42.

Item numbers with the suffix “a” correspond to those items in FIG. 2with the same number and no such suffix.

No structural detail of support tower 30 is shown, as any appropriateconstruction can be used. Tower 30 is in a position (on the central axis28) where only minimal restriction to flow of air in the passage 26 iscaused. Nevertheless, and also in the interests of minimal maintenanceand minimal restriction of air flow, a structure having a smoothexternal surface (as opposed to an open lattice structure) is preferred.One form (not shown) for tower 30 that is thought to be suitable is anupright tube. Such a tube and its foundation may be designed to haveadequate flexural stiffness (i.e. against lateral bending) without anyexternal support, or to be very slender and to have guy wires (or thelike) extending outwardly and downwardly from one or more points alongit to suitable anchor points.

The support structure must also be designed, and its material(s)selected, for adequate resistance to corrosion in the very wet and warmconditions in passage 26.

It is desirable that the axis of rotation of the impeller (eg 27) becoaxial with the passage 26 so that aerodynamic loads on the impellerblades 41 are substantially constant with time, to avoid possiblefatigue loading difficulties.

At the base of tube 33 is a gearbox 34 whereby shaft 29 is driven by aninput drive shaft 35 (not shown) which extends horizontally from gearbox34 to an electric motor 36 external to the tower 25. Shaft 35 is coaxialwith, and enclosed in a tube 37. Electric motor 36 drives impeller 27via shaft 35, gearbox 34 and shaft 29. By enclosing shafts 29 and 35 intubes 33 and 37, which are secured to gearbox 34, the need formechanical seals where shafts 29 and 35 enter gearbox 34 is avoided.This is desirable, given the hostile conditions of temperature andhumidity in cooling tower 25.

In the preferred embodiment, the electric motor 36 is part of a drivearrangement that can rotate impeller 27 at any of a range of speeds, sothat the airflow velocity, hence the cooling capability, of tower 25 canbe varied as required.

Impeller 27 is shown as having a swept diameter substantially the sameas that of the internal diameter of passage 26 at the height whereimpeller 27 is mounted, save for a suitable small operating clearance.Neck 38 of passage 26 is a possible place for location of the impeller27, because it enables provision of the smallest possible impeller 27.Also, because the mean velocity of airflow in passage 26 is highest atthe neck 38 of passage 26, experience suggests that that location bestlends itself to design for aerodynamic efficiency. However, impeller 27can be mounted at other heights in passage 26. In FIG. 4 impeller 27 isshown mounted at a lower height, to reduce the height (hence cost) ofsupport 30, albeit at the expense of a larger impeller 27 where thepassage 26 is to be substantially fully spanned.

The main reasons for making impeller 27 span essentially the whole widthof passage 26 are to enable the whole air flow to be enhanced and toavoid the potential problem of air recirculation within the passage 26.Additionally, the smaller the clearance between impeller 27 and innersurface 39 of hyperbolic wall 125, the higher the attainable aerodynamicefficiency of impeller 27. A suitable operating clearance between blades41 and surface 39 must always be provided, sufficient to ensure thatthere is no practical risk of contact between any blade 41 and surface39.

However, it is within the scope of the invention to provide an impeller(not shown) that is of significantly smaller diameter than the diameterof passage 26 at the vertical location of the impeller. In selecting theimpeller diameter, there is a balance to be struck between impellercost, and its power and structural requirements on one hand, and on theother hand, the need to avoid recirculation in the flow passage (eg 26)and to obtain satisfactory improvement in the overall thermodynamicperformance of the cooling tower in a satisfactory range of operatingconditions. The optimum compromise can be determined by developing, thenanalyzing and improving on trial designs, in known fashion.

Impeller 27 can be designed structurally, aerodynamically andaeroelastically using established design methods. Impeller 27 cansuitably be of low solidity, having for example 3 to 6 blades 41 ofcomparatively high aspect ratio (ratio of blade radial length to meanwidth between leading and trailing edges). That is, the impeller 27would in some cases resemble a modern “wind turbine” rotor more than thehigh solidity rotor of an agricultural “windmill” (for example). 3- or4-bladed impellers may often be found an appropriate choice. However, itis not intended here to in any way limit the scope of the invention torequiring a particular impeller design. The optimum blade design willdepend on the particular design parameters for a particular tower.

Blades such as blades 41, of considerable length and slenderness, cannow be designed and built, using modern materials and design. Thematerials—or surface treatments—would need to be such as to resist thewet, corrosive environment and in particular the impact of waterdroplets.

Blades 41 of impeller 27 are shown in FIG. 4 as being supported only atimpeller hub 42. However, it is also within the scope of the inventionto provide one or more intermediate supports for each blade as shown inFIG. 5. FIG. 5 shows a cooling tower 105 the same as cooling tower 25save that wire stays 43 extend outwards and downwards from a mast 44extending upwards from (and coaxial and arranged to rotate with) hub 45of an impeller 46. Each stay 43 is secured to one of the blades 47 ofimpeller 46. It would even be possible to provide for support of eachblade (such as 41 or 47) at its outer end by a small roller (not shown)running on a circular track (not shown) added to internal surface 40 ofthe wall 106.

FIG. 6 shows a cooling tower 48 (similar to cooling tower 25) with animpeller 49 having blades 50 which are mounted to a hub 51 byhorizontal-axis hinges 52, so that the blades 50 can move downwardlyfrom their operative positions to more nearly upright positions (shownin phantom lines) and vice versa. Suitable counterweighing (not shown)of each blade 50 radially inward of each hinge can enable the blades 50to be so balanced that in normal use they will extend substantiallyhorizontally and so that when the impeller 49 is not rotating they pivotto the lowered position. With this arrangement, resistance to purelyconvective flow of air (i.e. natural draft) is lowered when the impeller49 is not required to be used or is unserviceable. This arrangement canalso provide good access to the blades 50 from support 53 for inspectionor maintenance, when blades 50 are in the lowered position. As analternative to such counterweighing of the blades 50, mechanicalequipment (not shown) could readily be provided to actively raise orlower them as required.

A further possibility for reducing resistance to the natural draft whenoperation of the impeller 27, 46 or 49 is not required is to provide fordriving of any of the impellers 27, 46 or 49 at a lower-than-normalspeed that minimizes flow losses and requires only limited power inputto the impeller drive system. Still nother possibility is to provide forfeathering of the blades (41, 47 or 50) when equired, by known means.

In applying the invention, it is desirable to ensure that watercollected on the impeller blades (eg 41, 47 or 50) and flung outwardsfrom their tips centrifugally does not cause damage to the adjacentparts of the tower internal surface (eg 39 or 40). One way to avoid this(not shown) is to clad or otherwise protect the relevarit part of theinternal surface (eg 39 or 40) with a suitable protective skin. Suitablematerials for the skin could include stainless steel or rubber, forexample. Such cladding may also be designed to provide protection forthe tower structure against damage due to failure of a blade of therotor. Another possibility (not shown) is to provide conventional endplates or similar formations on the outer ends of the blades, tointercept such water and divert it downward.

Many variations may be made while remaining within the spirit and scopeof the invention. Taking cooling tower 25 as an example, the drive forimpeller 27 need not be purely electric as shown. It could be hydraulic,with a hydraulic motor at the base or top of support 30 and anelectrically driven pump (not shown) outside tower 25. Shaft 35 iseliminated.

It is also possible to provide several impellers within a cooling tower,if required. For example, each impeller could be mounted coaxially withthe other(s), an arrangement that would allow each to completely spanthe passage (eg 26), if required.

Still another possibility (not shown) is to support the impeller not, ornot only, on a support such as support tower 30, but from the towerstructure 125 (where this is determined to be structurally feasible) oreven from a separate structure (not shown) located partly above thetower. The latter possibility might require less downtime forinstallation of the system.

1-24. (canceled)
 25. A method for enhancing the performance capabilityof an existing wet type natural draft cooling tower, wherein the coolingtower: (a) is adapted including by size and cooling capacity in naturaldraft operation for use as a natural draft cooling tower in electricpower generating station application, (b) includes a structure definingan internal passage of circular cross-section for the upwardconvectional flow of an air stream therein from air inlet openings at ornear a lower part of the structure to an outlet opening at the top ofthe structure, and (c) contains heat transfer means in a lower part ofsaid passage for transferring heat from water supplied to said coolingtower to said air, and wherein said method includes the steps of:providing within said passage an impeller adapted when rotated at aspecified speed about an upright axis of rotation centrally located insaid passage in a specified operating condition of said tower toincrease the flow rate of air in the passage beyond an overall flow rateobtainable in identical operating conditions by natural draft alone,wherein said impeller when rotating at said specified speed spanssubstantially the entire diameter of said passage at the height of theperiphery of said impeller save for a suitable operating radialclearance between said impeller and an internal surface of said passage.providing support means adapted for supporting said impeller within saidpassage above said heat transfer means; and providing drive meanscapable of rotating said impeller at said specified speed.
 26. A methodaccording to claim 25 wherein said heat transfer means includes apacking structure having surfaces arranged to be wetted externally bysaid water so that heat is transferred from said water by evaporation ofa proportion of said water into said air and wherein said methodincludes the step of securing said support means to at least one of afoundation and a support structure of said packing structure of saidcooling tower.
 27. A method according to claim 25 wherein one saidimpeller only is provided in said passage.
 28. A method according toclaim 25 wherein said impeller is supported by said support means at aheight in said passage at least approximately corresponding to theminimum cross-sectional area of said passage.
 29. A method according toclaim 25 wherein said impeller is supported by said support means at aheight in said passage below a height at which said passage is ofminimum cross-sectional area.
 30. Apparatus for enhancing theperformance of a wet type natural draft cooling tower, said apparatusbeing adapted to use in a cooling tower that: (a) is adapted includingby size and cooling capacity in natural draft operation for use as anatural draft cooling tower in electric power generating stationapplication, (b) includes a structure defining an internal passage ofcircular cross-section for the upward convectional flow of an air streamtherein from air inlet openings at or near a lower part of the structureto an outlet opening at the top of the structure, and (c) contains heattransfer means in a lower part of said passage for transferring heatfrom water supplied to said cooling tower to said air, and saidapparatus including: an impeller adapted when rotated at a specifiedspeed about an upright axis of rotation centrally located in saidpassage in a specified operating condition of said tower to increase theflow rate of air in the passage beyond an overall flow rate obtainablein identical operating conditions by natural draft alone, wherein saidimpeller when rotating at said specified speed spans substantially theentire diameter of said passage at the height of the periphery of saidimpeller save for a suitable operating radial clearance between saidimpeller and an internal surface of said passage; support means adaptedfor supporting said impeller within said passage above said heattransfer means, and drive means capable of rotating said impeller atsaid specified speed.
 31. Apparatus according to claim 30 wherein saidsupport means is secured to at least one of a foundation and a supportstructure of said heat transfer means of said cooling tower. 32.Apparatus according to claim 31 wherein said heat transfer meansincludes a packing structure having surfaces arranged to be wettedexternally by said water so that heat is transferred from said water byevaporation of a proportion of said water into said air.
 33. Apparatusaccording to claim 30 having one impeller only.
 34. Apparatus accordingto claim 30 wherein said support means is adapted to support saidimpeller at a height in said passage at least approximatelycorresponding to the minimum cross-sectional area of said passage. 35.Apparatus according to claim 30 wherein said support means is adapted tosupport said impeller at a height in said passage below a height atwhich said passage is of minimum cross-sectional area.
 36. Apparatusaccording claim 30 wherein said impeller has a hub and a plurality ofblades secured to and in use extending generally radially from said hub.37. Apparatus according to claim 36 wherein said blades are secured tosaid hub via pivot means such that each blade is able to be pivotedbetween an operating position, assumed when said impeller is rotating atsaid specified speed, and a further position in which an outer end ofsaid blade is lower and radially inward of a position occupied by saidouter end when said blade is in said operating position.
 38. Apparatusaccording to claim 38 including means whereby each blade assumes saidfurther position when said impeller is stationary.
 39. Apparatusaccording to claim 36 further including protection means securable to aninternal surface of said passage at a position such that when saidimpeller is rotating at said specified speed, said protection means isadjacent to the outer end of each said blade and adapted to limit damageto said internal surface due to flinging of moisture from said blades.40. Apparatus according to claim 36 wherein each said blade has aformation at a radially outer end of said blade adapted to limitflinging of moisture collected on said blade onto an internal surface ofsaid passage when said impeller is in said operative position. 41.Apparatus according to claim 30 wherein said drive means includes anelectric motor.
 42. Apparatus according to claim 41 wherein saidelectric motor is outside said cooling tower structure and arranged torotate said impeller via a gear train enclosed in a casing in saidcooling tower structure.
 43. Apparatus according to claim 41 whereinsaid electric motor is operable when required as a generator so that ifsaid impeller is rotated by a natural draft within said cooling towerenergy can be extracted from said generator.
 44. A wet type naturaldraft cooling tower for cooling a liquid and having a cooling capacityvariable by a user, said cooling tower: (a) being adapted including bysize and cooling capacity in natural draft operation for use as anatural draft cooling tower in electric power generating stationapplication, (b) including a structure defining an internal passage ofcircular cross-section for the upward convectional flow of an air streamtherein from air inlet openings at or near a lower part of the structureto an outlet opening at the top of the structure, (c) containing heattransfer means in a lower part of said passage for transferring heatfrom water supplied to said cooling tower to said air, and (d) includingapparatus according to claim 30 for increasing the cooling capacity ofsaid cooling tower when in operation.
 45. A method for enhancing theperformance of a power generation plant in which: steam is passedthrough a turbine which drives an electric power generator and saidsteam is condensed in a condenser; cooling water for said condenser iscirculated through said condenser and a wet type natural draft coolingtower; said method including the steps of: adding to said cooling towerapparatus for enhancing the performance of said cooling tower, saidapparatus being apparatus according to claim 30; and operating saidapparatus.